There are many applications in which it is desirable to drill a curved bore in an object. For example, in orthopedic surgery, a number of procedures require a surgeon to secure tissue to bone by using stitches that extend through holes made in the bone; these procedures would benefit greatly if apparatus were commercially available that would allow a curved bore to be efficiently formed in the bone. Instead, the procedure typically employed requires that two angled bores be drilled in the bone, with the hope that the straight bores will intersect so that a curved surgical needle can be forced through the bore without breaking or jamming. To accommodate the curvature of the needle, the straight bores must be larger in diameter than is desired. In addition, there is often limited working space available where the holes must be drilled, making it difficult to maneuver a drill to produce the two straight holes from opposed angles. Of course, there are many industrial processes that would also benefit if low cost apparatus were commercially available that could produce smoothly curved bores in an object. Accordingly, the applications of such apparatus are not in any way limited to the medical field.
A solution to the problem of producing a curved bore is disclosed in two earlier U.S. Pat. Nos. 4,941,466 and 5,002,546, issued to the inventor of the present invention. In the first of these patents, a curved bore drilling apparatus and method are disclosed in which two driven shafts are provided with flexible shaft sections, each having a cutting tip. A semicircular curved drill guide attached to a pivotally mounted swing arm loosely engages and carries each flexible shaft and cutting tip. Two linkage rods couple the drill guides to a push rod that advance the cutting tips so that they simultaneously swing toward each other in intersecting 90.degree. arcs. The push rod is advanced by moving a pivotal handle relative to a stationary handle. In a second embodiment, the drill guides are simultaneously rotated toward each other by a worm and pinion drive actuated by the operator.
U.S. Pat. No. 5,002,546 discloses several different embodiments of apparatus for producing a curved bore using various machining processes in addition to the cutting tips. The apparatus disclosed for supporting the cutting tips and drive mechanism is shaped like a handgun; a trigger is mechanically coupled to various alternative linkages for advancing the cutting means to form the curved bore.
A significant problem with the apparatus for drilling a curved bore disclosed in these two patents relates to an interference between the two cutting tips that occurs when the cutting tips are swung toward each other to meet at about the center of the curved bore. Clearly, it is desirable that the bore be smoothly completed at its center or median point; yet, advancing both cutting tips simultaneously to meet at the center of the bore, as disclosed in this prior art, can cause the two cutting tips to be damaged as their cutting faces rotate against each other and can leave a rough circumferential lip at the median point, because neither cutting tip passes that point. One solution to this problem not disclosed in the prior art patents is to separately advance the cutting tips so that first one crosses over the median point in the bore and is then backed up before the other is advanced past the median point. In this manner, the two cutting tips never contact each other, but both rotate with their curved guide past the median point to complete a smooth curved bore in the object. Since the prior art does not disclose or suggest this technique, it clearly also fails to disclose any mechanism suitable for accomplishing the task.
Using two levers to independently advance the opposed cutting tips through their respective arcs at different times would achieve the desired goal, but is neither a very elegant nor a particularly practical solution to the problem. Ideally, the apparatus for drilling a curved bore should be capable of operation using only one hand, without requiring the user to manipulate separate control levers to advance each cutting tip. Manipulating separate levers to advance the two cutting tips at different times would likely require both hands and would be an unduly complex and difficult operation to repetitively complete, when producing multiple curved bores.
Another issue that is not disclosed in the prior art is the problem and solution for dealing with wear of the flexible drive cables and dulling of the cutting tips that will inevitably occur from time to time. A related issue concerns the need for producing different size and different radius bores without requiring that a different integral drive device be provided for producing each size and radius bore. The design of the apparatus for producing curved bores disclosed in the above-noted references does not readily facilitate replacement of the flexible drive cable and cutting tips, nor does it disclose a mechanism for changing the cutting tips and curved guides as appropriate to produce different size or different radius bores, while continuing to use the same rotational drive and advancement mechanism. Provision for coupling different radii curved guides or different diameter cutting bits housed in removable cartridges with a common drive mechanism offers a cost-efficient solution to this problem.
Because the radius of curvature defined by the path of the bore produced by the apparatus can be relatively small, e.g., less than 0.5 in., the flexible cable driving the cutting bit is forced through a correspondingly small radius of curvature. The point of attachment of the cutting tip or bit to the flexible cable is an area of substantially reduced flexibility in the cable, and unfortunately, is also a point of great stress. It has been observed that any breakage of the flexible cable during use of the curved bore drilling apparatus is more likely to occur adjacent the cutting tip than elsewhere. Accordingly, it is clear that some modification of the prior art apparatus is desirable to extend the useful life of the flexible cable.